1. Field of the Invention
This invention relates to electrostatic spray deposition techniques for applying an electrolyte material to a substrate, such as for a solid oxide fuel cell, wherein a precursor including the electrolyte material is discharged from a spray nozzle that has a collar positioned about the nozzle, for applying a relatively thin layer of the electrolyte material on the substrate surface.
2. Discussion of Related Art
Conventional electrostatic spray deposition (ESD) methods and apparatuses have been used to apply an electrolyte material to an anode structure of a solid oxide fuel cell (SOFC), but the conventional methods and apparatuses currently produce state-of-the-art electrolyte layers each having a thickness of 30–40 μm.
With conventional designs, one disadvantage is that materials and structures used to construct SOFC components cannot operate effectively at temperatures lower than 900° C.–1000° C., which is a temperature range at which conventional SOFC components operate. Yttria-stabalized zirconia (YSZ) is typically used as an electrolyte material because of its chemical stability and strength properties. However, even at a temperature of 1000° C., the specific ionic conductivity is relatively low, for example about 0.1 Ω−1.cm−1, and thus a thickness of the electrolyte layer must be relatively small. At higher operating temperatures of a conventional SOFC, a grain-boundary morphology connected with segregation, sintering, etc., can vary and thus reduce long-term and cycling stability of the conventional SOFC. In the industry, there is considerable effort to define and manufacture ceramic materials suitable for an intermediate temperature (IT) SOFC that has better stability than that of a conventional SOFC. One important requirement for improving a SOFC cell structure is to minimize an overall voltage loss of the SOFC.
Various conventional SOFC designs may be classified as electrolyte-supported, anode-supported and cathode-supported. The cathode-supported design is rarely used. In a planar cell the supporting cell component must provide sufficient mechanical strength to span a cell width, which is typically 10–20 cm. Thus the supporting cell component should be thicker than the other two components, and may even be thicker than an inter-connect (IC) layer if the IC layer is not designed for structured support, for example an IT-SOFC having a thin metallic foil for the IC layer. An electrolyte-supported SOFC usually has YSZ disks of approximately 100 μm thickness, on which relatively thin electrodes, each about 10 μm thick, are screen-printed. At an operating temperature of 1000° C. such an electrolyte layer thickness is tolerable, but at an operating temperature of 600–800° C., the electrolyte layer thickness must be much less, for reasons discussed below.
In a tubular design developed by the business entity Siemens-Westinghouse, the cathode is used as the supporting layer and thus has a cathode thickness of approximately 2 mm, which easily causes excessive polarization at relatively high current densities. The polarization is much less for an anode of comparable thickness, at least with hydrogen as a fuel. Anode-supported cells normally include a pre-fabricated relatively thick anode, on which a relatively thin electrolyte layer is deposited. The anode of an anode-supported SOFC is usually pre-sintered at a relatively low temperature to strengthen the anode, without significant shrinking. The YSZ electrolyte is then slurry-coated and sintered to the required temperature. The cathode is also slurry coated and sintered in a separate step.
Table 1 identifies properties of materials used in various cell components of a conventional tubular SOFC. The higher operating temperatures, around 1000° C., of the conventional SOFC limits the number of materials available for the cell components, because of a need to satisfy stringent criteria for chemical stability in oxidizing and reducing environments, for chemical stability of contacting materials, for conductivity, and for thermo-mechanical compatibility.
TABLE 1Specifications of SOFC Components in Tubular SOFCComponentConventional PropertiesAnodeNi/ZrO2 cermet (Y2O3 stabilized ZrO2)Electrochemical Vapor Deposition (EVD) or Slurrydeposition (EVD expected to be replaced by anodesintering)Thermal Coefficient of Expansion (TEC) 12.5 × 10−6cm/cm° C.~150 μm thickness20–40% porosityCathodeSr or Ca doped lanthanum manganite (SLM)Extrusion, sintering~2 mm thicknessTEC 11 × 10−6 cm/cm° C.Expansion from room temperature to 1000° C.30–40% porosityElectrolyteYttria (8 mol %) stabilized ZrO2 (YSZ)EVDTEC 10.5 × 10−6 cm/cm° C.Expansion from room temperature to 1000° C.30–40 μm thicknessCell InterconnectMg doped lanthanum chromitePlasma sprayTEC 10 × 10−6 cm/cm° C.~100 μm thickness
The physical limitations of current materials make apparent a need to develop cells with composition of oxides and metals that operate at intermediate temperatures in a range of 600–800° C.
Conventional SOFC designs make use of thin film concepts where films of electrode, electrolyte, and inter-connect material are deposited on one another and sintered, to form a cell structure. The state-of-the-art YSZ electrolyte in a SOFC operating at 1000° C. must be about 25–50 μm to keep the ohmic loss to a level comparable to that of the liquid electrolyte in a conventional PAFC. In manufacturing the tubular SOFC, dense YSZ layers of about 40 μm thickness are often fabricated by an Electrochemical Vapor Deposition (EVD) method, as well as by tape casting and other ceramic processing technologies.
A lower limit of the thickness of a YSZ electrolyte layer or another ceramic membrane is in part a function of the production process such as EVD, tape casting or other processes. The electrolyte layers deposited not only should be very thin and 100% dense, but should also have uniform composition and optimal microstructure. The electrolyte film should have sufficient mechanical strength to withstand the thermal stresses occurring due to start-up, shut-down, and other temperature swings during operation. As the thickness is reduced, the microstructure of the film becomes more important to adequately reduce ohmic resistance. It is believed SOFC with a thin-film electrolyte having a grain size of 100 nm or less can produce an overall electrolyte resistance at an acceptable low level.
With the IT-SOFC, whether or not using current electrolyte materials such as YSZ, there is a need to reduce resistance or ohmic losses that occur across mixed ionic-electronic conducting electrodes as well as ionic conducting electrolyte. Main ohmic losses are related to the electrolyte. Thus, there is an apparent need to reduce a thickness of the electrolyte layer. When the electrolyte layer is a relatively thin film, such as having a thickness of 5–15 μm according to this invention, its resistance at intermediate temperatures is comparable to, or less than, that of a conventional electrolyte layer having a thickness of 30–40 μm, and operating at 900–1000° C. There is a need to reduce an electrolyte layer thickness to 5–10 μm, or perhaps less, for SOFC operation at 600–800° C. To maintain IT-SOFC power densities well above those of the high-temperature SOFC, it may be necessary to reduce the thickness of the YSZ electrolyte layer to only a few micrometers.